1. Field of the Invention
The present invention relates to a camera equipped with an automatic focusing system and, more particularly, to an AF (auto-focus) single lens reflex camera.
2. Description of the Prior Art
Conventionally, a camera is proposed having an AF (automatic focusing) system which carries out the automatic focus adjustment based on a plurality of focus detection results. For example, in Japanese Patent Laid-open publication Nos. 56-78811, 57-128307 and 62-125311, the focus detection is carried out for a plurality of number of times to provide a plurality of defocus amounts. Then, the focus adjustment is carried out using the plurality of defocus amounts. However, in the prior art AF system, there are a number of disadvantages as explained below.
When the defocus amounts obtained in the present cycle and the previous cycle (herein, previous cycle means one cycle before the present cycle) have different signs (plus and minus) and, at the same time, none of the defocus amounts obtained in the past three subsequent cycles are in infocus condition, these focus detected results are disregarded and not used in the control. For example, in the case where the aiming object has a low luminance or low contrast, the focus detection is carried out for a plurality of number of times to obtain an average defocus amount with which the lens drive is controlled. However, even if an infocus detection is obtained during the series of focus detections for obtaining the average, such an infocus detection information is buried in the average focus amount. Thus, it takes long time before the infocus condition is obtained. Also, when the object is moving towards or away from the camera to generate a monotonously increasing or decreasing defocus amounts, an average of such defocus amounts will not provide the true defocus amount and, therefore, it takes long time before the infocus condition is obtained.
In the conventional AF system, the algorithm for a continuous mode (a mode in which the lens drive is constantly controlled in compliance with the moving object) is the same regardless of the presence and absence of the shutter release request. When the shutter release request is present to take a photography of a moving object, there is a time lag between a moment when the final focus detection is carried out and a moment when the shutter is actually release. Therefore, the focus adjustment effected at the time of shutter release based on the final focus detection will not result in an accurate focus adjustment.
In the conventional AF system, the focus adjusting mode, which is selectable between a single lock mode (a mode in which the focus adjustment condition is maintained once the infocus condition is obtained) and the continuous mode, is not switched automatically before and after obtaining the infocus condition.
In the conventional AF system, the focus adjusting mode, which is selectable among the single mode, the continuous mode, and the average mode (a mode in which the infocus condition is obtained based on an average of a plurality of defocus amounts), is not selected automatically.